Hydrophobicity and aggregate stability in calcareous topsoils from fire-affected pine forests in southeastern Spain

Abstract

Soil hydrophobicity is known to enhance runoff responses to rainstorms and to increase soil aggregate stability (AS). It has been widely reported for acidic soils particularly under burnt, but also unburnt pine forests following dry periods. Few studies have reported hydrophobicity from alkaline soils, but they have not established whether hydrophobicity also occurs in burnt or unburnt pine forests on alkaline soil. This study examines the wettability and stability of air-dry aggregates and their size fractions (<0.25, 0.25–0.5, 0.5–1 and 1–2 mm) taken from surface layers (0–2.5 and 2.5–5 cm depth) of alkaline, calcareous loamy soils. Four sites in southeastern Spain were sampled with comparable vegetation (>30-year-old Aleppo pine { Pinus halepensis} and associated shrub community), geology (limestone), soil type (Lithic Xerorthents), slope angle and aspect (5–8°SW). Included were three sites (A, B, C) burned, respectively, in 1998, 1999 and 2000, and one unburnt for >30 years (D). Hydrophobicity was detected in samples from all sites. Both spatial frequency and persistence of hydrophobicity (Water Drop Penetration Times (WDPT) ranged from 10 to 600 s), however, was lower than reported from studies of acidic soils under pine. This might be associated with a lower susceptibility of alkaline soils to hydrophobicity development and/or the comparatively low biomass production in the region. Probably because it had been most recently affected by severe fire, spatial frequency of hydrophobicity was higher at site B (53% of samples), compared to A, C and D (6%, 33% and 10% of samples, respectively). In contrast to some previous studies, the finest size fraction of the samples consistently had the highest degree of hydrophobicity. Degree of hydrophobicity was positively correlated with organic matter (OM) content ( r=0.714). It is speculated that fine, interstitial hydrophobic organic matter accumulating in the finest sieve fraction contributes to this enhanced hydrophobicity. As shown in previous studies on acidic soils, aggregate stability increased with hydrophobicity ( r=0.897 in the fraction 0.25–2 mm) for the samples investigated here. This elevated stability occurs despite an already relatively high level of aggregate stability amongst all samples investigated. Hydrophobicity observed at the study sites was not spatially contiguous and it may therefore enhance overland flow and slope wash over only short distances for most, except the very high intensity rainstorms that occur in the region. The increased stability of hydrophobic soil aggregates against slaking, however, may counter an otherwise enhanced susceptibility to erosion.